In order to improve the theoretical thermal efficiency of a spark ignition gasoline engine, increasing the geometrical compression ratio of the engine is effective. For example, Patent Document 1 teaches a high compression spark-ignition direct-injection engine with a geometrical compression ratio of 14 or higher.
For example, as shown in Patent Document 2, the combustion of compressing and igniting a lean air-fuel mixture is known as a technique improving both of the exhaust emission and the thermal efficiency. In an engine performing the compression ignition combustion, increasing the geometrical compression ratio improves both of the compression end pressure and the compression end temperature, and is thus advantageous in stabilizing the compression ignition combustion.
Patent Document 3 teaches performing EGR and enriching the air-fuel ratio in accordance with the operating mode of an engine in transition from compression ignition combustion to spark ignition combustion, thereby reducing knocking.